Henry Ott Consultants

The most important thing that I can say about slots in ground planes, is don't have them!  If you do have slots, no traces can cross over them.  If a trace does cross over the slot ask yourself this question: Where is the return path for the current?   Remember the fundamental principal of EMC, "return currents locally and compactly, through the smallest loop area possible."  If everyone would follow this principal, a great many of our EMC problem would go away or at least be minimized.
The lowest impedance signal return path is in a plane directly under the signal trace.  Slots in ground planes divert the ground current  flow (see figure), thereby producing a high ground impedance (Inductance) and a ground plane voltage drop that is the cause of increased emissions from the cables connected to the board.  In addition, ground plane slots will significantly increase  the crosstalk between traces that cross over them, and the larger loops formed by the return current paths will radiate.

The table below shows measured values of ground plane voltage with and without a slot in the ground plane.  The voltage measurements were made between two point on the ground plane one inch apart and directly underneath the trace.  The slot was cut perpendicular to the direction of current flow, as shown in the figure, and half way between the measuring points.  The entry under "holes" represents a linear pattern of fifteen holes each 0.052" in diameter (oriented perpendicular to current flow) covering a linear distance of one inch.    This is representative of an array of holes for vias or thru-hole component leads.  The voltage measurements were made with a 10 MHz, 3 nS rise time clock signal flowing down the trace and returning in the ground plane.  As can be seen the array of holes did not increase the ground plane voltage, but the slot did increase the voltage by a factor of as much as five in the case of the 1 1/2" slot.

Finding these problems, after-the-fact, on a buried ground plane that cannot be seen is a difficult and time consuming problem.  There is, however, a very simple way to initially check the quality of the ground plane.  When you have printed circuit boards made, ask to have one set etched but not laminated.  That way, you can look at the unlaminated ground plane and see if there are problems very quickly.  It would be a good practice to do this with all new designs.
 
 

  © 2001/2000 Henry W. Ott

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